Why three-phase power preferred over single-phase?

Single-phase power vs three-phase power

The electrical energy generated, transmitted and distributed in the form of three-phase power. But the homes and small premises connected with single-phase power.

Most of the time you will find that three-phase power is more preferred over single-phase power. Because, the three-phase machinery is more efficient than single-phase.

But why it is like that? Why three-phase power preferred over single-phase??? and What is the basic difference between three-phase and single-phase power.

You will be able to clearly understand the difference between three-phase and single-phase power and you will also get the idea why three-phase power is preferred over a single-phase.

Single-phase power

To understand the single-phase power, consider a generator with one winding and a permanent magnet. The winding has two terminals (A and A1). A permanent magnet is rotating inside the generator and it is driven by some external source like a turbine or prime mover.

single-phase generator
single-phase generator

When this magnet will start rotating, a sinusoidal voltage will get induced across the terminal of this winding.

If you connect a resistor across the terminals of the winding and make close path, the register will start taking current. And if we draw the waveform for current, it will look like the below figure. Here find that the current is in phase with the voltage.

waveform single-phase generator
waveform single-phase generator

“In phase” means that both are voltage and current start at the same time and reach their peak at the same time and get zero at the same time.

Now to get the instantaneous power, simply multiply the voltage with current and the resultant power waveform will look like sinusoidal waveform.

waveform resultant power single-phase generator
waveform resultant power single-phase generator

The funny thing about single-phase power is that the average power is one-half of its peak value. And also, the power output is not constant.

Two-phase system

Now let’s say using the previous single-phase generator, I added one more winding with terminal B and B1. Please note the winding B and B1 displaced 90-degree from winding A-A1.

The interesting part of this arrangement is that when the magnet is in the position shown a figure, the voltage across winding A-A1 is maximum or we can say it is at its peak and voltage across winding B-B1 is almost zero.

This is because the flux only cuts across the terminal in slot A and A1. So of course, when the magnet will rotate by 90-degree mechanically, the voltage across winding B-B1 will reach its positive peak and the voltage across winding A-A1 will be zero.

Therefore, we can say that these two voltages are out of phase by 90-degree. This simply means that one voltage will reach its peak before 90-degree than the other.

two-phase generator
two-phase generator

Now let me connect identical register across both the winding and current IR1 and IR2 will start flowing through resistors. These currents are in-phase with their respective voltage and hence they are also out of phase with each other by 90-degree.

waveform two-phase generator
waveform two-phase generator

The instantaneous power of both the resistor is as shown. Here you can notice that, when the power output of register R1 is zero, the power output of register R2 is maximum and vice versa.

If we add the instantaneous power of both the phases, we will find that the resultant power is constant and equal to the peak power PM of one phase.

In simple words, the power output of two phases generator is constant and better than a single-phase generator. Now you might have already got Why we use three-phase power.

Three-phase power

Consider a 2-phase generator. But this time, we’ll add one more winding C-C1 and we’ll place these three windings 120-degree apart from each other as shown below figure.

three-phase generator
three-phase generator

When the magnet will rotate, an identical voltage will get induced across all the three windings. As we have placed winding 120-degree apart from each other, the voltage induced in three phases will also be out of phase by 120-degree.

Let me make things clearer, when the magnet is in the position shown in above figure voltage across winding A-A1 is maximum, when the magnet rotates by 120-degree the voltage across winding B-B1 is maximum and when magnet rotates 240-degree from its initial position the voltage across winding C-C1 gets maximum.

Now let’s connect identical register across all the three windings current IR1, IR2, and IR3 will start flowing through resistors. These currents are in-phase with their respective voltage and hence they are also out of phase with each other by 120-degree.

Again, doing the same procedure, we did for the two-phase generator and find that the power output we are getting by adding the instantaneous power of all three phases is constant and it is 1.5 times the peak power of one phase.

waveform resultant power three-phase generator
waveform resultant power three-phase generator

So, here we have achieved constant and more power by simply adding two extra windings to a single-phase generator and this is the reason why three-phase power is more preferred over single-phase power.

Summary

  • So, to summarize this topic the single-phase power output is not constant and the average power is half than its maximum power.
  • Two-phase power output is constant at every instant and the average power is equal to the maximum power of one phase.
  • Three-phase power is also constant at every instance and the average power is 1.5 times the maximum power of one phase.

So, I hope this will make all the concepts about single-phase and three-phase power clear. if you found this article helpful do share it with your friend and you will find many more articles on this website.

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